How does a transistor amplify current or voltage?

[Ratchit]

Brownout,

They have various weaknesses. Most are too simple to be dependable.

I was hoping you would enumerate the failures of the diagram in the attachment I sent.

It's exactly what I said before. I've re-read what I wrote, evidently you haven't.

Agreement is what is important.

My statement is correct, as explained below.

As long as we are clear about a couple of things. Vbi = Vn + Vp and Vbi is cancelled across the diode, not the junction.

A PN junction is more than just a depletion region. vbi is cancelled by vn and vp during equalibrium and during forward bias. Nothing you've written has shown otherwise. Va changes and total depletion voltage changes, but vbi is still cancled, as it is nothing more then the intrinsic voltage, and does not change.

Agreed, as long as you realize that the cancellation is across the whole diode, and the constant Vbi across the junction does not change.

In other words, the voltage in the depleation region is reduced to vbi - va, under the condition of forced applied voltage. vbi is a constant, however, and does not change. Hence, the voltage across the diode is, from the perspective of the steady state voltage, equal to -va.

Good, that is what I want agreement on. Actually the forward bias voltage is +Va to make Vbi-Va, but I won't quibble. And "the voltage in the depleation region", which encompasses the PN junction, but not the whole diode, is another name for the junction voltage Vj, right? Then Vj = Vbi-Va, so when Va is 0, Vj = Vbi. Therefore, we have the b-e terminals at 0 volts and the internal PN junction voltage at Vbi.

The reason is because the change of barrier potential is identically equal to the voltage across the whole diode, the junction voltage ( as defined by Sedra and Smith ). This is bacause of the resaons given above.

Do I still understand you to think that the voltage across the whole diode to be the same as the junction voltage? The above sentence is a little confusing. The junction voltage (Vbi-Va) and voltage across the whole diode Va are two separate voltages.

Ratch

 

[BrownOut]

Agreed, as long as you realize that the cancellation is across the whole diode, and the constant Vbi across the junction does not change.

The constant vbi across the depletion layer doesn't change, and is cancelled by the charge potential in the bulk.

Good, that is what I want agreement on. Actually the forward bias voltage is +Va to make Vbi-Va, but I won't quibble. And "the voltage in the depleation region", which encompasses the PN junction, but not the whole diode, is another name for the junction voltage Vj, right? Then Vj = Vbi-Va, so when Va is 0, Vj = Vbi. Therefore, we have the b-e terminals at 0 volts and the internal PN junction voltage at Vbi.

Your last reference calls it junction voltage. Every other reference calls it depleation layer voltage. I prefer to use the more universal convention. Anyway, -va comes from doing the math, and shows the voltage is opposite of vbi. From the perspective of biasing the BE junction , it is +va. When va is zero then voltage across the diode is zero. When va is non zero, voltage across the diode, va, is equal to the reduction of depletion layer voltage. Or equivilently, the change in depletion layer voltage is reflected across the diode.

Do I still understand you to think that the voltage across the whole diode to be the same as the junction voltage?

That's what I've called it, not intended to confuse.

The junction voltage (Vbi-Va) and voltage across the whole diode Va are two separate voltages.

I would say delpletion layer voltage and voltage across diode are two different voltages, just to be more consistant with the majority of literature.

 

[Claude Abraham]

Claude,

OK, we agree on Vbi. Small changes in Vbe make large changes in Ie/Ic due to the exponential relationship.

I like to think of the voltage being the cause, because, as I was trying to explain to Brownout, the Vbe lowers the junction voltage, thereby allowing the diffusion forces to push more charges across the junction. The b-e terminals are usually driven by a current supply, but the b-e diode already has plenty of charges that cannot go anywhere due to the barrier voltage. So Vbe will accomodate to whatever is necessary to make the current exist. I consider this a backdoor method to adjust the nonlinear Vbe to the correct value, and the causal reason for voltage control.

I explained my reasons why I believe Vbe is causal. It directly affects the internal and separate junction voltage, and it does not matter if it happens before or after something else. I also agree that Ie/Ic are important, because they are the reason for the bJT's existence. You will disagree that Ib is necessary to bring Vbe to the junction, but is not the control. However, it is an indicator of output current.

Yes, charge is one way to look at it, but to move charge requires voltage or diffusion. Remember what the prof said about a BJT resembling a FET at low signal levels before secondary effects kick in? Does anyone have a quibble about a FET or a tube being a voltage control device?

Yes, in fact, a lot of transistor texts do not get involved in whether a BJT is CC or CV.

Whatever works best, as long as you treat FET's and tubes the same way.
Ratch

Once again, you just state that Vbe makes Ie/Ic change w/ no support. To you it doesn't need proof because it's too obvious. Then you claim that Vbe allows diffusion forces to puish more charges across the junction. Charges naturally push against one another. Voltage does not "push" charges, charges push charges. But to get charges to move in an orderly uniform manner, an E field is set up to provide drift. But stored charges exert a counter force to said E field, setting up a "barrier".

The key is the following statement by you "So Vbe will accomodate to whatever is necessary to make the current exist". But now you're treating Vbe as accomodating to meet imposed terminal conditions. You are now agreeing with me. I've been stating forever that Vbe is forced to whatever new value is needed to accomodate existing conditions. In other words, in your very own words, Vbe is incidental, important, but nonetheless incidental. I've said the same thing.

If Vbe is driven by the source of stimulus & forced to a value to accomodate internal & external conditions, then it cannot be causal. It does not force anything, internal or external. When you say that "Vbe will accomodate", my response is "way to go, you now get it". Accomodation, not causality, is what I've been trying to convey all along. Again, the sequence of events is all important. It matters whether or not Vbe changes before or after Ie/Ic. For Vbe to change after Ie/Ic rules out any chance of Vbe being causal.

So let me summarize. An increase in Ie/Ib/Ic cannot take place w/o an eventual increase in Vbe. Vbe absolutely must increase so that equilibrium conditions are achieved. Vbe is an accomodation, as you've correctly stated, that cannot be overlooked or avoided. It is an inevitable consequence of maintaining charge balance & meeting equilibrium conditions. Vbe adjusts to its new value after Ic has already begun to increase. Vbe is not the cause of Ic increasing, but is an unavoidable consequence & requirement for the change, just as is Ib. Ib is not causal either, because Ib does not directly contribute to Ic. But an increase in Ic cannot take place w/o an increase in Ib. Ib & Vbe indirectly participate in Ic. Ie is the controller. To be really precise, Sue controls everything.

Maybe the whole debate is over the definition of "causal". I'll ask you to please define the term. My understanding is that A is the cause of B if A's existence is independent of B, & A influences the behavior of B, making B behave in a manner it would not behave if A were not present. Is there other definitions? We seem to have universal concensus now on bjt internal operation. Your accomodation principle is sound by me, & my guess by others as well. Apparently the word "causal" means different things to different people. Please explain your concept of causality.

After 260 postings, great progress has been made. Now we must define causality. Cheers.

 

[BrownOut]

Remember what the prof said about a BJT resembling a FET at low signal levels before secondary effects kick in?

That's a grossly incorrect statement. The operation of BJT's and FET's are completely different, no matter how low level the signals are.

 

[Ratchit]

Brownout,

The constant vbi across the depletion layer doesn't change, and is cancelled by the charge potential in the bulk.

Whatever the "charge potential" of the bulk means. At equilibrium, the Vj = Vbi, at forward bias, Vj = Vbi-Vbe .

Your last reference calls it junction voltage. Every other reference calls it depleation layer voltage. I prefer to use the more universal convention.

Every other reference? Neudeck calls it the junction voltage.

Anyway, -va comes from doing the math, and shows the voltage is opposite of vbi.

When you do the math, you can see that from the loop equations Vbe and Vbi have the same polarity according to K's voltage law. See figs 2.6 of my previous attachment. But there is no need to quibble about that.

From the perspective of biasing the BE junction , it is +va. When va is zero then voltage across the diode is zero.

Yes, when Va or Vbe is zero, then surely the voltage across the diode is zero.

When va is non zero, voltage across the diode, va, is equal to the reduction of depletion layer voltage.

Good so far.

Or equivilently, the change in depletion layer voltage is reflected across the diode.

Yes, Vbe across the diode reduces Vbi by Vbe, giving Vj = Vbi-Vbe. This Vbe causes the current to increase by reducing the junction voltage Vj, thereby giving the rationale that voltage is what controls current in a diode.

That's what I've called it, not intended to confuse.

I would say delpletion layer voltage and voltage across diode are two different voltages, just to be more consistant with the majority of literature.

Good, then I think we are on the same track with that point.

That's a grossly incorrect statement. The operation of BJT's and FET's are completely different, no matter how low level the signals are.

The prof never said otherwise. He was pointing out that a BJT acts functionally like a voltage control device for very low signals.

Ratch

 

[BrownOut]

Every other reference? Neudeck calls it the junction voltage.

Sedra and Smith does not. The other references we've looked at does not.

When you do the math, you can see that from the loop equations Vbe and Vbi have the same polarity according to K's voltage law. See figs 2.6 of my previous attachment. But there is no need to quibble about that.

No, they are opposite by the math I've shown, and by evey reference. The loop equations say "vn -va + vp"

Yes, Vbe across the diode reduces Vbi by Vbe, giving Vj = Vbi-Vbe. This Vbe causes the current to increase by reducing the junction voltage Vj, thereby giving the rationale that voltage is what controls current in a diode.

No, the reduction in depletion voltage is the same as across the diode. vbe does not reduce vbi, as vbi is a constant (the depletion region potential at equalibrium). When the depletion voltage is reduced, that value is reflected across the diode. In other words, vbe is equal to the reduction of depletion voltage. vbi does not change.

The prof never said otherwise. He was pointing out that a BJT acts functionally like a voltage control device for very low signals.

He should define "very low signals" For all signals I've seen, it acts the same.

 

[Ratchit]

Claude,

Once again, you just state that Vbe makes Ie/Ic change w/ no support. To you it doesn't need proof because it's too obvious.

Not true. I quoted this equation from Sedra many times: Ic = Is*exp(Vbe/Vt).

Then you claim that Vbe allows diffusion forces to puish more charges across the junction. Charges naturally push against one another. Voltage does not "push" charges, charges push charges. But to get charges to move in an orderly uniform manner, an E field is set up to provide drift. But stored charges exert a counter force to said E field, setting up a "barrier".

If I said attract or repel instead of "push", would that offend your sensibilities? Should I have said that the electric field set up by the voltage repels the charges trying to diffuse across the junction? When bunch of the same charges get together, they naturally form a electric field that repels the same charge and attracts the opposite charge. The charges initially try to diffuse across the junction to where they are wanted. Eventually a built-in voltage Vbi forms across the junction due to uncovered charges. This repels the charges that are trying to cross the junction and eventually stops the diffusion current at equalibrium. Vbe lowers the junction voltage Vj so that more charges can diffuse across the junction, thereby increasing the diffusion current. Did I get that right?

The key is the following statement by you "So Vbe will accomodate to whatever is necessary to make the current exist". But now you're treating Vbe as accomodating to meet imposed terminal conditions. You are now agreeing with me. I've been stating forever that Vbe is forced to whatever new value is needed to accomodate existing conditions. In other words, in your very own words, Vbe is incidental, important, but nonetheless incidental. I've said the same thing.


The key is the following statement by you "So Vbe will accomodate to whatever is necessary to make the current exist". But now you're treating Vbe as accomodating to meet imposed terminal conditions. You are now agreeing with me. I've been stating forever that Vbe is forced to whatever new value is needed to accomodate existing conditions. In other words, in your very own words, Vbe is incidental, important, but nonetheless incidental. I've said the same thing.

If Vbe is driven by the source of stimulus & forced to a value to accomodate internal & external conditions, then it cannot be causal. It does not force anything, internal or external. When you say that "Vbe will accomodate", my response is "way to go, you now get it". Accomodation, not causality, is what I've been trying to convey all along. Again, the sequence of events is all important. It matters whether or not Vbe changes before or after Ie/Ic. For Vbe to change after Ie/Ic rules out any chance of Vbe being causal.

So let me summarize. An increase in Ie/Ib/Ic cannot take place w/o an eventual increase in Vbe. Vbe absolutely must increase so that equilibrium conditions are achieved. Vbe is an accomodation, as you've correctly stated, that cannot be overlooked or avoided. It is an inevitable consequence of maintaining charge balance & meeting equilibrium conditions. Vbe adjusts to its new value after Ic has already begun to increase. Vbe is not the cause of Ic increasing, but is an unavoidable consequence & requirement for the change, just as is Ib. Ib is not causal either, because Ib does not directly contribute to Ic. But an increase in Ic cannot take place w/o an increase in Ib. Ib & Vbe indirectly participate in Ic. Ie is the controller. To be really precise, Sue controls everything.

Maybe the whole debate is over the definition of "causal". I'll ask you to please define the term. My understanding is that A is the cause of B if A's existence is independent of B, & A influences the behavior of B, making B behave in a manner it would not behave if A were not present. Is there other definitions? We seem to have universal concensus now on bjt internal operation. Your accomodation principle is sound by me, & my guess by others as well. Apparently the word "causal" means different things to different people. Please explain your concept of causality.

After 260 postings, great progress has been made. Now we must define causality. Cheers.

OK, I will to explain what I mean by causality, and why I and others believe Vbe is causal to BJT operation. Causality is the principle of cause and effect. Vbe is causal to BJT current because Vbe the only thing that changes the junction voltage, and thereby BJT current. The fact that you can force Vbe to be a correct value with a current generator does not abrogate the fact that Vbe controls the junction voltage (Vj = Vbi - Vbe). The barrier voltage at equilibrium, also called the built-in voltage Vbi, is set during transistor manufacture, and does not change significantly. Changing Vbe is the only way you can directly change Ie/Ic. I know what you are going to say already. I can change Ic by changing Ib. Yes, you can do it. But what you are really doing is eventually changing Vbe, and Vbe is really changing the current as shown by Sedra's equation above, which does not show Ib as a parameter. So Ib is changing Ie/Ic indirectly by going through Vbe. That disqualifies Ib from being causal. So to summarize, I consider a BJT to be functionally CC and causally VC due to the fact that Vbe and nothing else is directly controlling Ic/Ie.

Ratch

 

[Claude Abraham]

Claude,


Not true. I quoted this equation from Sedra many times: Ic = Is*exp(Vbe/Vt).


If I said attract or repel instead of "push", would that offend your sensibilities? Should I have said that the electric field set up by the voltage repels the charges trying to diffuse across the junction? When bunch of the same charges get together, they naturally form a electric field that repels the same charge and attracts the opposite charge. The charges initially try to diffuse across the junction to where they are wanted. Eventually a built-in voltage Vbi forms across the junction due to uncovered charges. This repels the charges that are trying to cross the junction and eventually stops the diffusion current at equalibrium. Vbe lowers the junction voltage Vj so that more charges can diffuse across the junction, thereby increasing the diffusion current. Did I get that right?


OK, I will to explain what I mean by causality, and why I and others believe Vbe is causal to BJT operation. Causality is the principle of cause and effect. Vbe is causal to BJT current because Vbe the only thing that changes the junction voltage, and thereby BJT current. The fact that you can force Vbe to be a correct value with a current generator does not abrogate the fact that Vbe controls the junction voltage (Vj = Vbi - Vbe). The barrier voltage at equilibrium, also called the built-in voltage Vbi, is set during transistor manufacture, and does not change significantly. Changing Vbe is the only way you can directly change Ie/Ic. I know what you are going to say already. I can change Ic by changing Ib. Yes, you can do it. But what you are really doing is eventually changing Vbe, and Vbe is really changing the current as shown by Sedra's equation above, which does not show Ib as a parameter. So Ib is changing Ie/Ic indirectly by going through Vbe. That disqualifies Ib from being causal. So to summarize, I consider a BJT to be functionally CC and causally VC due to the fact that Vbe and nothing else is directly controlling Ic/Ie.

Ratch

As I've stated, there is an alpha factor in the E-M eqn, which you perpetually omit on purpose. Ic = alpha*Ies*(exp((Vbe/Vt)-1). Again, this is a functional relation.

Secondly, E fields are not set up by voltages. but rather by both current & voltage. Whenever an E field imparts energy to a charge carrier, that E field loses the same amount, & gets replenished by a current. Also, if a current source is used to drive a bjt, of course it will adjust Vbe to the correct value. That is what a current source does. However, that circuit is still a CC circuit. When we say a device is CC. it is understood that the source providing the current will output the necessary voltage. It is still current control because current is the directly controlled variable, w/ voltage indirect.

Likewise, a VC device such as a FET, gets driven by a CV source. This CV source will output whatever current is needed to charge the g-s cap. It's still VC. Vgs is the directly controlled variable, & Ig is indirect, the counterpart of the bjt. Is that hard to understand?

For the umpteenth time, you keep re-iterating that Vbe causes Ie to change, eventually changing Ic. But the fact that Ie/Ib change ahead of Vbe does not seem to dissuade you. You explicitly state cause & effect, yet you have Vbe as the cause, chronologically lagging Ie, its effect. Cause must take place before the effect. I gave a definition of causality above & asked you to define what you mean by the term.

Your "Vbe will accomodate" was correct, but now you still invoke causality, which is invalid on its face. People with this causality viewpoint cannot bring themselves to accept anything defined in terms of current, except for maybe magnetic devices. You remind me of a defense trial lawyer. The lawyer has 1 objective, to acquit his client. Any evidence that weakens his defense is evidence he tries to suppress. He is always looking for ways to exclude damning evidence by claiming it was illegally obtained, impeaches the credibility of eyewitnesses, tries to strectch previous rulings/precedents in a manner friendly to his client. Objectivity is not what defense counsel is seeking. About 6 yrs. ago, I served on a trial jury. Throughout the trial we heard evidence that the defendent burglarized a house. The defense counsel went after every witness' credibility, questioned hard evidence regarding relevance, etc. Finally, the defendent took the stand. Under cross examination, he finally broke down. He burst into tears & confessed, just like on that old tv show "Perry Mason". This made the jury's job too easy.

Anyway, during their closing statements, the defense counsel, asked for a verdict of not guilty! He claimed that the case presented by the prosecutor was not that strong! He acted as if the open confession never occurred! I am now reminded of that scenario in this thread.

The laws of science positively affirm that Vbe is not the cause of Ie/Ib. That position has been shown to be incompatible w/ sequence of events when scrutinized during transient conditions. In equilibrium conditions, all variables have settled to their final value & it is not possible to say which changed first, which is consequential. This "voltage is causal" position, is just plain dogma adhered to with force & conviction equal to that of a religion. It is not factually objective.

But no amount of factual presentation will dissuade the causality believers, so I'll just let it go at that. There will never be 100% consensus on anything. Some are pre-disposed to view V as the cause of I. I've heard this theory since I was an undergrad in the 1970's. Thirty-five years later, no circuits, fields, or physics book ever refers to one as the cause of the other. Attempts to show the inconsistencies in their position are met w/ dogma & denial. Good day.

 

[MrAl]

Hello again,


You know i was doing something else and i realized that my example of collector current control didnt seem to sink in or else i missed something.

The initial setup is where we have a fixed Vbe and fixed Ibe and set Ic to some lowish value to make sure the transistor is fully into saturation.
Next, the collector current is increased, and that pulls the transistor out of saturation. That's the operation for a basic oscillator that depends on saturation of the magnetics to oscillate (various names).

Now it's pretty clear that the collector current is doing all the forcing action, but we can stretch it to say that Vbe being fixed is no longer able to sustain saturation so the device moves out of saturation, but that's pretty much trying to force the definition of Vbe control and i just cant see it. The collector current is increased and that makes the transistor behave the way it does. If the collector current was not increased this action would not happen. All the while Vbe stays constant and never changes throughout the experiment. You could say that changing Vbe means a different collector current would allow the transistor to pull out of sat at a different level, but what if we never change Vbe, as for this experiment.

With this experiment it seems to me that it is very very hard to try to claim voltage control. Very hard.

 

[Claude Abraham]

Hello again,


You know i was doing something else and i realized that my example of collector current control didnt seem to sink in or else i missed something.

The initial setup is where we have a fixed Vbe and fixed Ibe and set Ic to some lowish value to make sure the transistor is fully into saturation.
Next, the collector current is increased, and that pulls the transistor out of saturation. That's the operation for a basic oscillator that depends on saturation of the magnetics to oscillate (various names).

Now it's pretty clear that the collector current is doing all the forcing action, but we can stretch it to say that Vbe being fixed is no longer able to sustain saturation so the device moves out of saturation, but that's pretty much trying to force the definition of Vbe control and i just cant see it. The collector current is increased and that makes the transistor behave the way it does. If the collector current was not increased this action would not happen. All the while Vbe stays constant and never changes throughout the experiment. You could say that changing Vbe means a different collector current would allow the transistor to pull out of sat at a different level, but what if we never change Vbe, as for this experiment.

With this experiment it seems to me that it is very very hard to try to claim voltage control. Very hard.

Very good insight, Mr. Al. Moving in & out of saturation cannot be explained w/ VC, nor w/ CC. Here, QC is the only description equipped to handle the scenario. Pulling in & out of saturation requires delay time, rise time, storage time, & fall time, as well as reverse recovery charge. QC addresses these issues, but VC does not, & just to be fair, neither does CC.

All scientific eveidence amassed points to CC externally, & QC internally. That has been my position for 3 decades, & I've used it in industry w/ no problem at all. I design a lot of discrete analog circuitry. CC & QC have never let me down.

Great illustration about the oscillator. Here is another along that line. In the 80's, when we used bjt power devices as switches for SMPS, remember the technique known as "proportional base drive"? A transformer winding couples the drive pulses to the bjt b-e terminals. A control network sensed the bjt c-e voltage, & drive was limited when the Vce value entered saturation. Let's say device A is driven w/ proportional base drive, PBD, & device B is not, allowing B to fully saturate. Both devices are switched on hard. It is now time to turn both off. Device A comes out of saturation faster because it was never allowed to go too deep into saturation. Both devices, A & B, have a Vbe of zero volts. But the one w/ less stored charge can transition faster. It is the charge stored, not Vbe that determines this. As I have been saying for decades, CC externally, Qc internally. Also, Schottky diodes are sometimes connected from base to collector to prevent the bjt from going too deep into saturation. You've all likely seen this done. Same idea.

So, bjt behavior does not merely depend on present terminal conditions, but on past history as well. To see this you have to subject the bjt to fast pulses, & measure the transient response of the several variables. I stated this earlier. The bjt has "memory" to an extent. The 3 Ic eqns I gave earlier do not acknowledge time/frequency. The QC eqns do. Observing device behavior under dc/steady state/equilibrium conditions is useless. Only transient conditions can unlock the secrets of device behavior.

Your oscillator example is a great illustration. Thanks for sharing it as well as your many contributions to this forum.

 

[MrAl]

Hello Claude, and thank you very much...and thanks for your contributions also...


Yes the Baker Clamp...thanks for reminding me. Why would we possibly need this? Pure and simple: charge (again).

For some reason Ratchet wants to restrict his theory to a static model as might be used in voltage reference applications or as i think Brownout pointed out the log converter type applications. Sure, we could restrict ourselves to those apps which then makes the voltage control ideology self-fulfilling. In that case, i think ill restrict my ideology to collector current controlled only so that i can never use voltage control nor even base current control <little chuckle>.
Come to think of it, from now on maybe i'll just shine a really really bright light onto all of my transistors, so bright that some of it gets through to the base-emitter diode and energizes it via the photovoltaic effect, which boosts electrons. Oh wait a minute, that would be charge control again Or do we say that it creates a voltage first...since the electrons are being controlled it would seem that charge control is still at work, although some would argue that the voltage and current are both there at the same time.

I am not, strictly speaking, disagreeing with Ratchet (yet), but i need to see a diagram which clearly outlines his theory, or else since he doesnt want to do that, then how about an experiment i can perform which will show me positively that voltage control is the only form of true control. That would also do it for me.

Ratchet:
An experiment that shows positively that voltage control is the only control would be nice since you dont want to do a diagram. I'd appreciate seeing this. Thanks. I also have to thank you for your patience here.

I hope all you guys appreciated the little humor with this post...

 

[Ratchit]

Brownout,

Sedra and Smith does not. The other references we've looked at does not.

What's in a name? Whatever your references call it, don't you think that junction voltage Vj is a good name for the voltage across the junction?

No, they are opposite by the math I've shown, and by evey reference. The loop equations say "vn -va + vp"

You have quoted a term, not an equation. Would you like to try again? Since I do not have access to the material you have, but you do have the attachment from Neudeck, why don't you show where Neudeck went wrong in his presentation.

No, the reduction in depletion voltage is the same as across the diode. vbe does not reduce vbi, as vbi is a constant (the depletion region potential at equalibrium). When the depletion voltage is reduced, that value is reflected across the diode. In other words, vbe is equal to the reduction of depletion voltage. vbi does not change.

We already agreed on that. I never said that Vbe reduces Vbi, which is a constant. I said that Vbe reduces the junction voltage, as in Vj=Vbi - Vbe.

He should define "very low signals" For all signals I've seen, it acts the same.

Yes, I wish he had provided a more complete explanation.

Ratch

 

[BrownOut]

What's in a name? Whatever your references call it, don't you think that junction voltage Vj is a good name for the voltage across the junction?

Not when it's properly called the depletion layer voltage.

You have quoted a term, not an equation. Would you like to try again? Since I do not have access to the material you have, but you do have the attachment from Neudeck, why don't you show where Neudeck went wrong in his presentation.

Actually I quoted the right side of the equation, and it contains three terms.

We already agreed on that. I never said that Vbe reduces Vbi, which is a constant. I said that Vbe reduces the junction voltage, as in Vj=Vbi - Vbe.

Wrong again, here is what you wrote:

Quote by Ratcit Yes, Vbe across the diode reduces Vbi by Vbe

That's the problem when post something, it becomes part of the permanent record, you can't go back later and say you didn't say something that you’re on the record saying. But anyway, the voltage across the diode ends up being the same as the reduction in depletion layer voltage, which is due to excitation. That is because vbi is cancelled by vn and vp during forward conduction, as well as during equilibrium.

 

[Ratchit]

Claude,

As I've stated, there is an alpha factor in the E-M eqn, which you perpetually omit on purpose. Ic = alpha*Ies*(exp((Vbe/Vt)-1). Again, this is a functional relation.

Are you trying to introduce a model again? You know what I already said about that. Everyone knows that Ic=alpha*Ic. Alpha is just another constant factor. How does that pertain to this discussion?

Secondly, E fields are not set up by voltages. but rather by both current & voltage. Whenever an E field imparts energy to a charge carrier, that E field loses the same amount, & gets replenished by a current.

I know an E-field is caused by a accumulation of charges. And you can cause a E-field to by energizing a capacitor with a voltage, which causes a accumulation and depletion of charges on the capacitor plates. The movement of charges defined as current does not cause a E-field, unless it causes the charges to accumulate. The movement of charges cause a magnetic field. Again, what does this have to do with what we are talking about?

Also, if a current source is used to drive a bjt, of course it will adjust Vbe to the correct value. That is what a current source does. However, that circuit is still a CC circuit. When we say a device is CC. it is understood that the source providing the current will output the necessary voltage. It is still current control because current is the directly controlled variable, w/ voltage indirect.

Functionally speaking, yes. But since the current source is controlling Vbe indirectly, and Vbe is adjusting the junction voltage, and the junction voltage is controlling the current directly, I and others consider Vbe to be the causal factor. I thought I explained that in my last post. It is voltage that is in the Sedra equation, not current. So voltage is the direct controlling element.

Likewise, a VC device such as a FET, gets driven by a CV source. This CV source will output whatever current is needed to charge the g-s cap. It's still VC. Vgs is the directly controlled variable, & Ig is indirect, the counterpart of the bjt. Is that hard to understand?

I understand what you are saying, but it does not detract from what I said about why Vbe should be considered the direct control of Ic/Ie.

For the umpteenth time, you keep re-iterating that Vbe causes Ie to change, eventually changing Ic. But the fact that Ie/Ib change ahead of Vbe does not seem to dissuade you. You explicitly state cause & effect, yet you have Vbe as the cause, chronologically lagging Ie, its effect. Cause must take place before the effect. I gave a definition of causality above & asked you to define what you mean by the term.

Yes, whether Ie/Ib change before or after Vbe is not an issue. I explained how Vbe changes the junction voltage, which changes Ie/Ic. I already defined what I meant by casuality in my last posting. I cannot do better than that.

Your "Vbe will accomodate" was correct, but now you still invoke causality, which is invalid on its face.

I don't think it is, and I explained why, even if you don't accept it.

People with this causality viewpoint cannot bring themselves to accept anything defined in terms of current, except for maybe magnetic devices. You remind me of a defense trial lawyer. The lawyer has 1 objective, to acquit his client. Any evidence that weakens his defense is evidence he tries to suppress. He is always looking for ways to exclude damning evidence by claiming it was illegally obtained, impeaches the credibility of eyewitnesses, tries to strectch previous rulings/precedents in a manner friendly to his client. Objectivity is not what defense counsel is seeking. About 6 yrs. ago, I served on a trial jury. Throughout the trial we heard evidence that the defendent burglarized a house. The defense counsel went after every witness' credibility, questioned hard evidence regarding relevance, etc. Finally, the defendent took the stand. Under cross examination, he finally broke down. He burst into tears & confessed, just like on that old tv show "Perry Mason". This made the jury's job too easy.

I could and have said some thing about relevance about what you presented. But anyway, I explained my position sufficiently, even if you don't believe it.

Anyway, during their closing statements, the defense counsel, asked for a verdict of not guilty! He claimed that the case presented by the prosecutor was not that strong! He acted as if the open confession never occurred! I am now reminded of that scenario in this thread.

False confessions are a problem in law enforcement. But reminisces don't prove anything.

The laws of science positively affirm that Vbe is not the cause of Ie/Ib. That position has been shown to be incompatible w/ sequence of events when scrutinized during transient conditions. In equilibrium conditions, all variables have settled to their final value & it is not possible to say which changed first, which is consequential. This "voltage is causal" position, is just plain dogma adhered to with force & conviction equal to that of a religion. It is not factually objective.

And I believe that transistor technology shows that Vbe is the direct control of its current. And that from studying several equilibrium points, and the reasons why they occur at their values, it is still possible to determine what is directly controlling Ic. I bet that the Pope considered Galileo dogmatic too.

But no amount of factual presentation will dissuade the causality believers, so I'll just let it go at that. There will never be 100% consensus on anything. Some are pre-disposed to view V as the cause of I. I've heard this theory since I was an undergrad in the 1970's. Thirty-five years later, no circuits, fields, or physics book ever refers to one as the cause of the other. Attempts to show the inconsistencies in their position are met w/ dogma & denial. Good day.

At least nothing you presented thus far.

Ratch

 

[Ratchit]

Brownout,

Not when it's properly called the depletion layer voltage.

No, it is not. Vbi is the "depletion layer voltage". That does not change. The junction voltage changes according to Vj = Vbi - Vbe. Are you aware that you cannot find the term "depletion layer voltage" with a Google search?

Actually I quoted the right side of the equation, and it contains three terms.

Why don't you quote the whole equation including the left side.

Quote by Ratch: We already agreed on that. I never said that Vbe reduces Vbi, which is a constant. I said that Vbe reduces the junction voltage, as in Vj=Vbi - Vbe.

Quote by Brownout: Wrong again, here is what you wrote:

Quote by Ratch: Yes, Vbe across the diode reduces Vbi by Vbe

Quote by Brownout: That's the problem when post something, it becomes part of the permanent record, you can't go back later and say you didn't say something that you’re on the record saying. But anyway, the voltage across the diode ends up being the same as the reduction in depletion layer voltage, which is due to excitation. That is because vbi is cancelled by vn and vp during forward conduction, as well as during equilibrium.

Yes, I should have said Vj instead of Vbi, as I correctly wrote in the next sentence. I believe permanency is a good thing, don't you? Like in post #257, where you say "Across the entire junction, vj is vbe.", when you really meant to say Vj = Vbi - Vbe. I believe we agree on this.

Ratch

 

[BrownOut]

No, it is not. Vbi is the "depletion layer voltage".

That's true only at equilibrium -- no external excitation source.

The junction voltage changes according to Vj = Vbi - Vbe. Are you aware that you cannot find the term "depletion layer voltage" with a Google search?

I couldn't give a damn less. It's in semiconductor physics text books. I use physics texts to learn physics. I don't care what google says. It's also called barrier voltage.

Why don't you quote the whole equation including the left side.

Why should I? You don’t know what the equation is?

Like in post #257, where you say "Across the entire junction, vj is vbe.", when you really meant to say Vj = Vbi - Vbe. I believe we agree on this.

Nope, I meant to say "Across the entire junction, the reduction in the depletion voltage is vbe."

 

[Ratchit]

MrAl,

You know i was doing something else and i realized that my example of collector current control didnt seem to sink in or else i missed something.

The initial setup is where we have a fixed Vbe and fixed Ibe and set Ic to some lowish value to make sure the transistor is fully into saturation.
Next, the collector current is increased, and that pulls the transistor out of saturation. That's the operation for a basic oscillator that depends on saturation of the magnetics to oscillate (various names).

Hot flash! Increasing the collector current too much is what brings a transistor into saturation. Saturation occurs when Ib no longer controls Ic. A zero Ic is the cutoff point. How come no one but I pointed that out to you?

Were you trying to infer something about causal control of a BJT when it is not in the active region?

Ratch

 

[Ratchit]

Brownout,

That's true only at equilibrium

Barrier voltage = Vbi never changes, equilibrium or not, after transistor manufacture, except for slight temp changes.

I couldn't give a damn less. It's in semiconductor physics text books. I use physics texts to learn physics. I don't care what google says. It's also called barrier voltage.

OK, let's call it barrier voltage.

Why should I? You don’t know what the equation is?

I want to be sure we are talking about the same thing. Why do something only halfway?

Nope, I meant to say "Across the entire junction, the reduction in the depletion voltage is vbe."

Doesn't the equation say it more concisely?

Ratch

 

[BrownOut]

Barrier voltage = Vbi never changes, equilibrium or not, after transistor manufacture, except for slight temp changes.

That's false. The barrier voltage is bi only when no source is connected. A source will reduce the barrier voltage. Sedra and Smith correctly describes that.

I want to be sure we are talking about the same thing. Why do something only halfway?

You can look at the eqations just as easily as I can. The signs are what were important to my point.

EDIT: eqn 2.43 in your last attechment.

Doesn't the equation say it more concisely?

the equation is for the barrier potential, not across the diode. I'm taling about how the reducton in barrier voltage, barrier potential, depletion voltage, junction voltage, or whateve you want to call it, is the same as across the diode, or BE potential.

 

[Ratchit]

Brownout,

That's false. The barrier voltage is bi only when no source is connected. A source will reduce the barrier voltage. Sedra and Smith correctly describes that.

Vbi is the barrier voltage and is constant, whether a source is connected or not. The source will not reduce the barrier voltage, it will reduce the junction voltage Vj = Vbi -Vbe. Vj = Vbi only when Vbe = 0. I don't have the same Sedra book you have.

You can look at the eqations just as easily as I can. The signs are what were important to my point.

EDIT: eqn 2.43 in your last attechment.

Thank you, I did not know whether your were referring to my attachment something only you had. A forward biased Vbe is subtracted from Vbe, or the negative value added.

the equation is for the barrier potential, not across the diode.

Correct, it is the junction voltage, the voltage across a width as small as a wavelength of visible light.

I'm taling about how the reducton in barrier voltage, barrier potential, depletion voltage, junction voltage, or whateve you want to call it, is the same as across the diode, or BE potential.

Barrier or built-in voltage is fine. The junction voltage is the result of subtracting Vbe from Vbi. We appear to agree on that.

Ratch